1. Field of the Invention
This invention relates to a method of injection molding and moldings produced thereby.
2. Prior Art
The manufacture of various types of synthetic resin moldings is known using various suitable resin materials such as, for example, general purpose plastics, engineering plastics and thermoplastic resins and adopting appropriate molding methods such as, for example, compression molding, extrusion molding and injection molding. Due to the rapid penetration of electronic equipment e.g. facsimile terminals, word processors, small to medium size computers, and telecommunications terminals, there is an increasing demand for housings for this equipment made from synthetic resins. The advantages of housings made from synthetic resins are lighter weight and the possibility of one piece moldings. There is also a strong demand for larger size housings to be made in the form of synthetic resin moldings.
For such housings there is a requirement for a pleasing external appearance, with no appreciable inward bending, and a high molding accuracy with no appreciable outward bending. There is a strong requirement for lighter housings. Furthermore, it is desirable that the housings or other moldings do not require painting or other after treatments.
Various technologies for manufacturing moldings which meet these various requirements have already been proposed. For example, in order to manufacture lighter moldings, there exist well-known methods using synthetic resin materials in which a molten resin is mixed with a foaming agent and expanded moldings are produced by injection molding. It is, however, very difficult to manufacture thin walled moldings using these methods, and the external appearance of such moldings is poor. Various techniques have been proposed to compensate for this poor external surface appearance in which a hardened surface skin layer is formed and the inner section comprises an expanded layer. Other methods of manufacturing lighter moldings are also known in which hollow internal layers are introduced into the moldings. However, these methods are not appropriate for thin walled moldings. Also, the molding cycle is long and unsuitable for molding the above-mentioned housings.
Methods have also been proposed for producing hollow moldings in which the quantity of molten resin injected into the mold cavity is insufficient to fill the cavity and subsequently pressurized gas only or pressurized gas and molten resin are injected to fill the mold cavity. However, with these methods there is a tendency for the gas to pierce through the molten plastics surface. The formation of the desired molding is also hindered when gas only is injected under pressure and the injection of gas is not very carefully controlled. This lack of control renders these methods unsuitable for complex shaped moldings.
A method has been proposed in which the operating conditions are controlled so that molten resin and gas flow simultaneously into the mold space and hollow moldings are manufactured. However, even with this method gas will easily pierce the resin surface when, for example, resin materials are used which have a low viscosity in the molten state.
Ribs are frequently used as strengthening sections in order to allow a reduction in the wall thickness of moldings having a large surface area and to provide strength and rigidity to the resulting thin walls. However, it is well known that inward bending of the surface occurs in the ribbed sections of such moldings and the external appearance of these moldings is poor.